I. Field of the Invention
This invention relates to an improved process for the preparation of oxidation and corrosion resistant cobalt, iron, or nickel-based alloy powders from a plurality of constituents in powder form, by high energy mechanical alloying in an attritor-type mill, wherein at least one constituent is an active metal, such as aluminum, present as a substantial proportion of the total and is in elemental form.
II. Description of the Prior Art
U.S. Pat. Nos. 3,591,362 and 3,723,092 issued Mar. 1, 1968 and Mar. 27, 1973, respectively, to Benjamin describe mechanically alloyed metal powders and a process for their preparation, respectively; both of which are incorporated herein by reference in their entireties.
Illustratively, Benjamin in U.S. Pat. No. 3,723,092 describes the basic steps of the mechanical alloying process as, ". . . providing a dry charge of attritive elements and a powder mass comprising a plurality of constituents, at least one of which is a metal capable of being compressively deformed, subjecting the charge to agitation milling under high energy conditions in which a substantial proportion or cross section of the charge is maintained kinetically in a highly activated state of relative motion, and continuing the milling to produce wrought composite metal powder in which particles thereof have substantially the saturation hardness for the system involved."
Benjamin further teaches that the term "mechanical alloy" applies to the state which prevails in a composite metal particle produced in accordance with his invention wherein, ". . . a plurality of constituents in the form of powders, at least one of which is a compressively deformable metal, are caused to be bonded or united together by the application of mechanical energy in the form of a plurality of repeatedly applied compressive forces sufficient to vigorously work and deform at least one deformable metal and cause it to bond or weld to itself and/or to the remaining constituents, be they metals and/or non-metals, whereby the constituents are intimately united together and indentifiably codisseminated throughout the internal structure of the resulting composite metal particles."
Benjamin teaches that a large variety of alloy powders may be made by the mechanical alloying process in which at least one of the constituents is a readily compressively deformable metal comprising at least about 15% or more by volume of the total powder composition. Thus alloy systems ranging from the simple, e.g., lead-base, aluminum-base, copper-base, nickel-base, cobalt-base, and refractory metal-base, to the complex, e.g., the well known heat resistant alloys such as those based on the nickel-chromium, cobalt-chromium, and iron-chromium systems, may be mechanically alloyed. Both the simple and more complex alloys can be produced with uniform dispersions of hard phases, such as refractory oxides and refractory carbides, nitrides, and borides, by the mechanical alloying process. In the mechanical alloying of the more complex alloys, such as the well-known heat resistant alloys, Benjamin teaches that alloying constituents such as molybdenum, tungsten, columbium and/or tantalum, aluminum, titanium, and zirconium, may be added in their elemental form, or to avoid contamination from atmospheric exposure, as master alloys or metal compound additions wherein the more reactive alloying addition is diluted or compounded with a less reactive metal such as nickel, iron, cobalt, etc.
Benjamin teaches that it is important that the milling process be conducted in the dry state and that liquids be excluded from the milling environment since they tend to prevent cold welding and particle growth of metal powder and tend to promote the formation of flakes. Benjamin notes that inert gas media tend to enhance product particle growth and may be of assistance when powder mixtures containing active metals such as aluminum, titanium, etc., are being milled. Also oxygen up to about 1% by weight in excess of that added as a refractory oxide dispersoid, preferably not exceeding about 0.75% or about 0.5% oxygen is seen as beneficial.
In U.S. Pat. No. 3,877,930, issued Apr. 15, 1975, the entirety of which is incorporated herein by reference, Volin describes the use of organic interdispersion cold bonding control agents (ICBCA) in the mechanical alloying process. Volin teaches that mechanical alloying, particularly of non-dispersion strengthened alloys of the superalloy type, conducted in the presence of an argon-nitrogen atmosphere are susceptible to several problems. For example, oxygen retained in the particles of nickel-base superalloys tend to degrade metallurgical properties such as the tensile strength and creep ductility of those alloys. If oxygen is present to an excess, comminution of the powders dominates to such an extent as to virtually preclude the critical necessary cold bonding. In the absence of oxygen-bearing atmospheres, the powders either adhere irreversibly to the attriting elements and interior attritor surfaces or, depending upon the composition of the attriting elements and powder charge, may irreversibly cold bond to form undesirably large particles.
To facilitate the mechanical alloying of alloys such as the nickel-base superalloys, Volin teaches that it is necessary to establish a "control balance" through the use of an ICBCA. ICBCAs may be selected from such compounds as, for example, methane, ethane, propane, hexane, methanol, stearic acid, and oleic acid. These ICBAs are added in the range of not more than 2-3% with a range of 0.05 or 0.1 to 0.5% and up to 2% being deemed satisfactory. It is considered by Volin that the ICBCAs establish the "control balance" by virtue of their reacting with or being adsorbed on the surfaces of the powder particles thus causing occlusion of the particles during collisional events and that this inhibits metal-to-metal bonding.
U.S. Pat. No. 4,101,713, issued July 18, 1978, to Hirsch and Rairden, the entirety of which is incorporated herein by reference, teaches the use of the high energy mechanical alloying process to produce metal powders suitable for the plasma flame spraying of superalloy articles to impart improved high temperature oxidation and corrosion resistance. Hirsch and Rairden teach the use of master alloy powders of cobalt and aluminum as starting materials for flame spraying powders that have aluminum as a substantial consituent. Hirsch and Rairden also teach recovery of the milled powder by use of a raised perforated bottom plate.
Despite the prior art teachings, the production of alloy powders by the mechanical alloying process, wherein at least one of the constituent powders is an active metal, such as aluminum, present as a substantial proportion of the total and is in elemental form has been found to produce an inhomogeneous product and result in low yield of the small-sized powders particularly suitable for plasma flame spraying.
Therefore, it is an object of this invention to provide an improved method for producing mechanically alloyed powder wherein at least one of the starting constituent powders is aluminum or other active metal, present as a substantial proportion of the total and is in elemental form.
A further object of this invention is to provide an improved method for increasing the yield of small-sized mechanically alloyed powders suitable for plasma flame spraying applications wherein at least one of the starting constituent powders is aluminum, or other active metal present as a substantial proportion of the total and is in elemental form.
Another object of this invention is to increase the total yield of alloy powder recovered from attritor-type mills.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.